In the prior art, writing servo information onto a disk medium for positioning a head at an aimed track of a disk medium (Servo Track Writing: STW) is performed after the disk medium has been incorporated into the magnetic disk drive. In this regard, there is a problem that a spindle and a head actuator may oscillate in the STW operation, resulting in meandering servo tracks written due to the superimposed radial component of the vibrations. Accordingly, a method for writing servo information with a smaller error is disclosed in Japanese Patent Laid-open No. 9-320002. In this method, before a magnetic disk medium is assembled into a magnetic disk drive, servo information is written onto the magnetic disk medium in the state of a single plate by using a high precision spindle motor and a head actuator which is held so as to suppress its oscillation. The magnetic medium is assembled to a spindle of the magnetic disk drive after the STW operation is done.
In addition, methods for fixing a magnetic disk medium to a spindle hub with a suppressed eccentric error are disclosed in Japanese Patent Laid-open Nos. 1999-185445, 2000-268533 and 2001-10174.
In the case of the method where servo information is written onto a disk medium before the magnetic disk is assembled to a spindle of a magnetic disk drive, the disk's center of rotation in the disk drive must coincide with the center of the written tracks as accurately as possible.
To fix the disk medium with no eccentricity from the center of rotation of the spindle hub by using the method disclosed in Japanese Patent Laid-open No. 1997-320002, it is necessary to make the outside diameter of the spacer identical to the inside diameter of the disk medium. However, this is not feasible due to the high machining precision required for the spacer and disk medium. Even if the outside diameter of the spacer can be made equal to the inner diameter of the disk medium, another problem occurs. When pulling out the spacer, the disk medium may be shifted since the medium was in contact with the spacer.
In the method disclosed in Japanese Patent Laid-open No. 11-185445, there is a problem that since each positioning mechanism moves radially in the case of multi-disk medium, an attempt to narrow the space between disk media in a multi-disk drive is limited.
In the method disclosed in Japanese Patent Laid-open No. 2000-268533, since the respective actuators are bumped against disk media at different timings, positioning requires increased time. In addition, this method is still not free from the problem of deflecting the hub.
In the method disclosed in Japanese Patent Laid-open No. 2001-101741, if the air jet holes of the spindle hub, for each magnetic disk medium, are different in altitude, the magnetic disk medium waves vertically. In addition, if the air jet holes have different bore diameters, the magnetic disk medium is positioned eccentrically around the spindle hub. To prevent these situations, the air jet holes for each magnetic disk medium must be uniform in altitude and bore diameter but this is not feasible in view of the machining precision required to manufacture such a spindle hub.